Eat worms and train your immune system

Argh! Exclamation marks.

Here's an article in the New York Times marred by a false statement about evolution:

Since all instinctive behaviors have an evolutionary advantage or they would not have been retained for millions of years, chances are that this one too has helped us survive as a species.

There are plenty of reason why not every trait need to ever have been advantageous. Pleiotropic constraints is one that comes to mind. I wonder if there is anyone who have evidence that this should be different for behavioral traits.

The article, Babies Know: A Little Dirt Is Good for You, is actually quite intriguing, though. The gist of it is that the immune system needs to be trained and kept busy by the environment that it evolved to cope with, so eating worms is a great idea. Babies know this instinctively, which is why they eat dirt at the playground. My kids must have an excellent immune system, then. This theory better be correct.

Dr. Weinstock goes even further. “Children should be allowed to go barefoot in the dirt, play in the dirt, and not have to wash their hands when they come in to eat,” he said. He and Dr. Elliott pointed out that children who grow up on farms and are frequently exposed to worms and other organisms from farm animals are much less likely to develop allergies and autoimmune diseases.

Also helpful, he said, is to “let kids have two dogs and a cat,” which will expose them to intestinal worms that can promote a healthy immune system.

I do wonder, though, if the behavioral trait of eating dirt is adaptive, then why is the behavioral trait of parents to stop them from doing so adaptive, given that "all instinctive behaviors have an evolutionary advantage?"